Pluck the meat from the chicken and cut it into bite-sized pieces.
Drain the liquid from the white asparagus and cut them also into bite-sized pieces - remember to save the asparagus liquid.
Melt the butter in a pot over medium heat.
Add flour and whisk together with a whisk until the butter has absorbed the flour.
Pour the liquid from the asparagus into the pot while whisking vigorously.
Add the chicken stock gradually and whisk until you have a smooth sauce without lumps.
Add the milk gradually until you have the desired consistency - it should be more like a stew than a sauce.
When the sauce has the desired consistency, add the asparagus and chicken. Stir gently so the asparagus doesn't break up too much.
Season with salt, pepper, vinegar, and lemon juice and possibly a bit more asparagus liquid.
Warm the tarteletter according to the package instructions and fill them with the mixture.
Garnish with chopped parsley and serve immediately.